Electrical oscillator for generation of high frequencies



June 4, 1940. w; FLECHSIG 2,203,035

ELECTRICAL OSCILLATQR FOR GENERATION OF HIGH FREQUENCIES Filed Dec. 16, 1936 Illlu- IIII Ill |.l

INVENTOR Mina" [66/062, 7 v 752% ATTORNEYS Patented June 4, 1940 UNITED STATES ELECTRICAL OSCILLATOR FOR GENERA- TION OF HIGH FREQUENCIES Werner Flechsig,

Berlin- Charlottenburg,

Ger-

many, assignor to the firm Fernseh Aktien- Gesellschaft, Zehlendorf, near Berlin, Germany Application December 16, 1936, Serial No. 116,176 In Germany December 20, 1935 5 Claims.

The object of the invention is to provide an electric oscillator using high vacuum tubes, especially oxidecathode tubes, which easily starts oscillations at high and ultra-high frequencies.

Theinvention is based on the fact that the alternating voltage between grid and cathode lags behind the alternating voltage between anode and cathode. This phase angle, which has a maximum of 180 for long waves, is smaller for short waves because of the transit time of the electrons. At ultra-high frequencies, which may for instance correspond to a wave-length of about two meters, this lagging angle may be about since at these frequencies the cyclic period of the circuit approaches the electron transit time of the tube.

For a certain oscillator frequency a certain phase angle is a requirement given by the tube characteristics-naturally within certain limits. Consequently, the frequency of oscillation is not the same as that to which the resonant circuits of the oscillator are tuned, but a frequency which differs somewhat from this resonant frequency. Nevertheless, the oscillation will be sustained as long as the difference in frequencies stays small. If, however, a certain limit is passed the oscillation will. suddenly stop because the resonant circuits cannot oscillate with sufficient intensity at a frequency which is too far off their natural frequency, or can be made to oscillate only by very close coupling which leads to disturbing sound modulation. Consequently, it is not possible to cover a large range of frequencies with the conventional shortwave oscillators, and for smaller ranges of frequencies the readiness to oscillate depends largely on uncontrollable circumstances.

This invention is based on the recognition of the circumstances causing these phenomena and eliminates the mentioned disadvantages by application of auxiliary complex impedances which are disposed in the circuit of the regenerating currents and serve as phase shifters. These auxiliary impedances should have such properties that the oscillator frequency is identical with the natural frequency of the resonant circuits for at least one point of the frequency range. When determining the natural frequencies of the resonant circuits the inductance and capacitance of the wiring and of the tube electrodes are also to be taken into consideration. Thus, for this one point of the frequency range, the most favorable condition is obtained by the auxiliary components, as the resonant circuits can oscillate actually at their natural frequencies, since the phase requirement given by the oscillator frequency is fulfilled. A forcing of the resonant circuits consequently no longer takes place. The arrangement so strongly favors the desired frequencies that only in the rarest cases is it necessary to take measures against so-called wild or parasitic oscillations.

For the neighboring frequencies, however, the

most favorable conditions no longer exist. In any case, however, the difference between the oscillator frequency and the natural frequency of the resonant circuits is considerably smaller than without phase-shifting means. In this manner, it is readily possible to cover a comparatively large range of frequencies.

The drawing shows several modifications of the invention. Figure 1 shows a circuit in which several other inventive ideas are incorporated. Figure 2 shows an equivalent circuit. Figure 3 shows a characteristic curve which is intended to demonstrate the electrical conditions in the new circuit. Figure 4 is a vector diagram for the same purpose. Figures 5 and 6 are two further modifications of the invention in order to show the manifold possibilities of the new system.

The plate I of the tube shown in Figure 1 is connected to ground through a resonant circuit I, which consists of an inductor 2 and a variable capacitor 3 in parallel. The grid 4 is connected to ground through inductor 5, which is coupled to inductor 2. The cathode 6 is indirectly heated by the filament 1. This filament 1 is effectively grounded and has a choke coil 8, in each of its leads. tional regenerative circuits of this kind in that a choke coil 9 is disposed in the lead to the cathode 6.

The cathode 6 has a circuit capacitance to ground which is indicated by the dotted capacitor l0. Figure 2 shows the equivalent circuit, taking this capacitance into consideration. It may be clearly seen that the capacitance it), together with the auxiliary choke coil 9, forms a resonant circuit II.

The properties of this resonant circuit II are of determining influence on the operation of the oscillator, as may be explained with the aid of Figure 3. Figure 3 shows the reactive impedance of the resonant circuit II as a function of the frequency, and for simplicitys sake the effective losses are neglected. In such a graph, the first part of the curve, that is, the part between the zero point and the resonant frequency wrII, signifies an inductive reactance, and the succeeding part a capacitive reactance, as is known.

This oscillator differs from the conven- This second part is used as the range of operation.

The reasons therefor may be seen from the vector diagram, Figure 4, which shows the voltages for the balanced condition, whereby the resonant circuit I oscillates at its natural frequency. The vector BA signifies the alternating voltage between the point B (Figures 1 and 2) and the plate I, the vector BG representing the alternating voltage between the point B and the grid 4. These vectors are displaced by exactly 180, under the above-mentioned assumption that the resonant circuit I oscillates at its natural frequency, so that itrepresents a purely ohmic resistance.

The vector KB represents the voltage between the cathode 6 and the point B. Considering this voltage, it may be seen from the diagram that the resulting vectors KA and KG, which are the voltages between cathode 6 and anode l and between cathode 6 and grid 4 respectively, enclose a phase angle which is smaller than 180. If the vector KB has the correct magnitude and phase angle, that is, if the components of the circuit II are suitably chosen, such a phase angle may be obtained as is required for the optimum condition of oscillation in view of the electron transit time in the vacuum tube. The oscillator will then oscillate at the natural frequency of the circuit 1. The higher the frequency of the oscillator the smaller the lagging angle must be. The maximum of this angle is 180 for long waves.

As the circuit I acts as a pure resistance when in resonance, the plate current must be in phase with the vector BA. in order to give the voltage KB the shown direction, the circuit II through which the plate current also fiows must be substantially capacitively reactive. Therefrom, it may be seen that the natural frequency of the circuit II must lie below the range of oscillation of resonant circuit 1, and hence also below the range of oscillation of the oscillator.

A comparatively Wide range of frequencies can be covered with the above-mentioned means. If there are, however, especially severe requirements, a further improvement of the phaseshifting auxiliary impedance is necessary. As shown in Figure 8, the capacitive reactance of the circuit II decreases with increasing frequency whereas, as shown in the diagram, the vector KB should increase with increasing frequency in order to obtain a smaller phase angle corresponding to the relatively greater transit time of the electrons. The arrangement of Figure 5, which like Figure 1, represents, a Meissner regenerative circuit, but in somewhat modified form, shows by what means this may be accomplished. As means for phase shifting, not only is a fixed component used, but beside the choke coil 9 a further variable component, for example, a capacitor H, and it is practical to vary this capacitor simultaneously with the capacitor 3. By means of such a variabie combination of impedances, it is possible to substitute for the falling characteristic of the capacitive range in Figure 3 a rising characteristic. By suitable choice of the components of the complex impedance, especially of the variable component, it is possible to come as closely as desired to the theoretically required curve. Of course, the invention also includes the case Where the lag must be smaller than or even change into a lead. For this purpose, devices are used which deliver a difference in angle of zero or 90. -Th-is may be accomplished in the circuits shown by changing the polarity of the regenerative voltage or taking the voltage ofi of a series combination of ohmic resistance and inductance or capacitance, and amore accurate adjustment is made by variation of the complex impedance which is coupled with the tuning circuit.

If the requirements are not quite as severe it may be possible to do withoutthe variable component and the desired characteristic may be obtained in the known manner by combination of several fixed components, such as inductances, capacitances and resistances. If the requirements are very low, that is, if the apparatus is intended to cover only a narrow range within the range of ultra-high frequencies, a pure ohmic resistance as phase-shifting component may be suflicient, if this resistance is chosen according I to the given directions in view of the inherent eapacitances.

In the latter case, too great an inherent capacitance of the cathode to ground would be a disadvantage inasmuch as the vector KB of the diagram in Figure 4 would have an insuflicie'ntly small value because of the too great admittance of such a capacitance. This disadvantage may be eliminated by disposing chokes in the filament path, as is shown by chokes 8 in Figure 1. A further improvement can be obtained if the shielding and a possibly existing suppressor grid is connected directly to ground instead of to the oathode.

two of the most important oscillator circuits with the aid of the mentioned figures, may be applied in the same manner to other oscillator circuits, which generate high frequencies and which are equipped with resonant circuits. From the great number of circuits for this purpose, Figure 6 shows the circuit known as the Huth-Kiihn circuit. The same is also true for the Hartley circuit.

Although only indirectly heated tubes are mentioned above, it is, however, understood that the principal idea of theinvention may be also applied for directly heated tubes in the same manner.

The invention is not limited to circuits with inductive feedback coupling, but is also applicable in exactly the same manner to capacitive and other feedback couplings.

The field of practical application is the range of ultra-short waves below a limit of about 12 meters and especially below -8 meters. nomena, upon the conception of which the invention is based, also .exist for longer waves, but are The invention which has been. explained for The phel of less importance in that range, because the" resonant circuits can be forced easily on ac count of the slight deviation of the phase, angle from the limit of It is only in the range below 12 meters that this forcing becomes difficult and that oscillations will be entirely stopped.

Having thus described my invention, what is claimed is:

1. An oscillator comprising an electron discharge device having a cathode and plurality of electrodes, a circuit connecting said cathode to one of said electrodes, another circuit connecting said cathode to a different electrode and including a portion of said first circuit adjacent the cathode, one of said circuits being coupled to the other and including a circuit tunable within a predetermined frequency range, and capacitive reactive means in the common portion of said coupled circuits responsive to the varying of 'f said tunable circuit to render said means increasingly capacitively reactive as the frequency of said tunable circuit increases.

2. A high-frequency oscillator comprising an electron discharge device having a cathode and plurality of electrodes, a circuit connecting said cathode to one of said electrodes, another circuit connecting said cathode to a different electrode, said circuits having a portion eifectively in common and being coupled, said coupling including a resonant circuit whose cyclic period approaches the transit time of electrons in said discharge device, and means in said common circuit having a resonant frequency substantially lower, than the frequency of said resonant circuit for establishing a phase angle of substantially less than 180 between the voltage developed in one of said coupled circuits and that developed in the other of said coupled circuits.

3. A high-frequency oscillator comprising an electron discharge device having a cathode and plurality of electrodes, a circuit connecting said cathode to one of said electrodes, another circuit connecting said cathode to a different electrode, said circuits having a portion effectively in common and being coupled, said coupling including a tunable circuit adjustable within a frequency range whose cyclic periods approach the transit time of electrons in said discharge device, and means common to said coupled circuits and having a resonant frequency substantially lower than the tuning frequency of said tunable circuit for establishing a phase angle between the voltage developed in one of said coupled circuits and that developed in the other of said coupled circuits in accordance with the cyclic period of the frequency to which said tunable circuit may be tuned.

4. A high-frequency oscillator comprising an electron discharge device having a cathode, an anode and an intermediate electrode, a circuit including a resonant circuit connecting said cathode to said anode, said resonant circuit having a cyclic period approaching the transit time of electrons in said discharge device, another circuit coupled to said first circuit and connecting said cathode to said intermediate electrode and including a portion of said first circuit in common therewith, and means for establishing a phase angle of substantially less than 180 between the voltage developed in the anode circuit and that developed in the circuit of said intermediate electrode comprising a tuned circuit in the common portion of said coupled circuits, the resonant frequency of which tuned circuit is substantially lower than that of said resonant circuit.

5. An oscillator comprising an electron discharge device having a cathode and a plurality of electrodes, a circuit connecting said cathode to one of said electrodes, another circuit coupled to said first circuit and connecting said cathode to a diiferent'electrode and including in common with said first circuit the connection from said cathode, a tunable frequency-determining circuit in one of said coupled circuits, and a tuned circuit in said common connection from said cathode, said tunable circuit having a tuning range on the capacitive side of said tuned circuit.

WERNER FLECHSIG. 

